1. Field
The present invention relates generally to the field of communications, and more particularly to a method, apparatus, and system for efficient user-multiplexing in multiple access communication systems.
2. Background
In recent years, communication systems' performance and capabilities have continued to improve rapidly in light of several technological advances and improvements with respect to telecommunication network architecture, signal processing, and protocols. In the area of wireless communications, various multiple access standards and protocols have been developed to increase system capacity and accommodate fast-growing user demand. These various multiple access schemes and standards include Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA), Code Division Multiple Access (CDMA), and Orthogonal Frequency Division Multiple Access (OFDMA), etc. Generally, in a system which employs TDMA technique, each user is allowed to transmit information in his assigned or allocated time slots whereas an FDMA system allows each user to transmit information on a particular frequency that is assigned to that particular user. A CDMA system, in contrast, is a spread spectrum system which allows different users to transmit information at the same frequency and at the same time by assigning a unique code to each user. In an OFDMA system, a high-rate data stream is split or divided into a number of lower rate data streams which are transmitted simultaneously in parallel over a number of subcarriers (also called subcarrier frequencies herein). Each user in an OFDMA system is provided with a subset of the available subcarriers for transmission of information. The subset of carriers provided to each user in an OFDMA system can be fixed or vary, for example, in the case of Frequency-Hopping OFMDA (FH-OFDMA). Multiple access techniques in TDMA, FDMA, and CDMA are illustrated in FIG. 1. As shown in FIG. 1, the communication channels in FDMA are separated by frequencies in which a particular channel corresponds to a particular frequency. In a TDMA system, the communication channels are separated by time in which a particular channel corresponds to a particular time slot. In contrast, communication channels in a CDMA system are separated by codes in which a particular channel corresponds to a particular code.
In wireless systems, it is usually inefficient to guarantee a reliable packet transfer on every single transmission. The inefficiency is particularly pronounced in systems where underlying channel conditions vary drastically from transmission to transmission. For example, in an FH-OFDMA system, there is a wide variation in the received signal-to-noise ratio (SNR) between frames/packets, thus making it difficult and inefficient to guarantee a small frame error rate (FER) for each packet transmission. Such difficulty and inefficiency also apply to other communication systems which employ orthogonal multiple access techniques including, but are not limited to, TDMA, FDMA, and orthogonal CDMA, etc.
In such communication systems, a packet retransmission mechanism such as the Automatic Retransmission/Repeat Request (ARQ) scheme may be used to help lessen such inefficiency. However, this is done at the expense of higher packet latency since it takes longer on average for each packet to get through. In general, large packet latency may not be a significant problem for data traffic but could be detrimental to voice traffic or other types of applications that require low latency in transmission of information. Moreover, packet transmission latency is expected to increase as the number of users in the system continues to grow. Thus, to improve system capacity (e.g., based on system throughput or number of users that simultaneously use the system, etc.), transmission latency should be kept low or small.
In systems which employ ARQ scheme, there is a non-negligible additional delay associated with each transmission acknowledgment. In particular, it may take up to several packet transmission times before an acknowledgement (ACK/NAK) of a previously transmitted packet to come back. To improve the link utilization, an S-channel ARQ can be implemented in these systems. The term S-channel ARQ refers to the fact that there are S interlaces (or S interlacing packet streams) from a transmitter to a receiver in these systems. For example, a dual-channel ARQ is often implemented in these systems. As shown in FIG. 2, the term dual-channel refers to the fact that there are two interlacing packet streams from a transmitter to a receiver (denoted by solid lines and dotted lines in FIG. 2). It can be seen that in this type of system configuration, rather than waiting for the acknowledgement (ACK/NAK) to come back prior to sending the next packet, the transmitter continues sending packet whenever available and responds to the acknowledgement when it arrives. While resource utilization (e.g., link utilization) is improved in a dual-channel ARQ system, transmission latency continues to be an issue that needs to be addressed. For example, as illustrated in FIG. 2, when there is a transmission error (e.g., transmission of packet index #2 in slot index n+2), an acknowledgment of such error (e.g., NAK) is received in slot index n+3 and the respective packet is not retransmitted until one slot later (slot index n+4).
Accordingly, there exists a need for a method, apparatus, and system for reducing transmission latency in multiple access systems that employ packet retransmission mechanisms such as ARQ.